In recent years, from the viewpoint of global environmental conservation, independence from petroleum-derived resources is being studied, and various natural resources are receiving attention. Independence from petroleum-derived resources is also being attempted in the plastic field, and polylactic acids produced from lactic acid obtained by glucose fermentation are widely used in packaging materials and the like.
According to Non-patent Document 1, polylactic acids have high transparency, but are inferior in impact resistance, heat resistance, and hydrolysis resistance. Therefore, the use of polylactic acids in compacts by injection molding and the like is limited.
In addition, not only polylactic acids, but, as described in Non-patent Documents 2 and 3, PET (polyethylene terephthalate) and PC (polycarbonate), which are petroleum-derived general-purpose polymers, have insufficient durability because they are readily hydrolyzed at a high temperature and humidity or in an acidic or alkaline environment, and improvement thereof is desired.
Rosin is a naturally-derived product collected from, for example, pine-tree gum. Rosin is composed of a mixture of various terpene carboxylic acids. Of these carboxylic acids, abietic acid is known to be useful as a polymer material (for example, see Patent Documents 1 and 2). For example, Patent Documents 1 and 2 disclose the use of a rosin-modified phenolic resin and a rosin-modified epoxy acid resin, which are prepared by modifying a phenolic resin or epoxy resin with abietic acid at the end of the resin, as binders for paints and the like. However, these resins are petroleum-dependent materials because they include a phenolic resin or an epoxy resin as the main skeleton, and thus do not reflect the viewpoint of global environmental conservation.
In addition, polymers prepared by polymerization of abietic acid with a polyalcohol are also known (for example, see Patent Document 3). However, since the polymers described in Patent Document 3 can be randomly polymerized to cause gelation, such polymers do not form linear polymers having a high molecular weight. Accordingly, these polymers are not suitable for industrial applications such as compacts.